1. Field of the Invention
This invention relates to semiconductor wafer processing and more particularly to methods for patterning lines in a metal layer formed over a semiconductor wafer.
2. Background of the Technical Art
The production of packaged integrated circuits (I.C.'s) involves many processes. A key element of these I.C.'s is the semiconductor die or "chip" that is patterned in accordance with the circuit design. An important process in the production of I.C. die is the patterning of the metal layer(s) that form the interconnections between the various circuit components. These interconnections or lines can be both narrowly and widely spaced on a single die depending upon design requirements.
FIGS. 1a-1c illustrate a series of steps during a prior art patterning process of a metal layer formed over a semiconductor wafer. In FIG. 1a, a metal layer 10 lies over a substrate 12, preferably an oxide layer overlying a semiconductor wafer, such as a silicon wafer 13. A mask 14 comprising a pattern of photoresistive material is formed over the metal layer 10 by techniques well known to those skilled in the art of I.C. photolithography. After the mask 14 has been formed, the metal layer 10 is etched by either a wet or dry etch process as is well known to those skilled in the art to form the metal lines shown in FIG. 1b. FIG. 1c shows the resultant pattern of metal lines 16 after removal of the mask 14. Although the lines shown in FIG. 1c all have vertical sidewalls for purposes of illustration, this is not necessarily a consistent or ideal result for all line patterns.
In FIGS. 2a-2c, several variations in the profile shape of densely packed etched metal lines are shown. FIG. 2a shows closely packed lines 17a with vertical sidewalls formed over substrate 19a, FIG. 2b shows closely packed lines 17b with sloped sidewalls formed over substrate 19b, and FIG. 2c shows closely packed lines 17c with reentrant sloped sidewalls formed over substrate 19c. Problems occur when the lines are sloped as shown in FIGS. 2b and 2c. The sloped sidewalls of lines 17b as shown in FIG. 2b can cause shorting between the lines by direct contact of the sidewalls such as at point 18, or by a particulate 20 that can be trapped between two adjacent sidewalls. The reentrant sloping of lines 17c of FIG. 2c can cause problems in subsequent layering, because the deposited material comprising the next layer may not completely backfill under the sidewalls, so that holes or voids may then be present in the deposited layer, which may lead to failure of the I.C. chip. In order to avoid the problems nonvertical sidewalls cause in closely packed metal lines, it is typically best to etch densely spaced metal lines with vertical sidewalls as shown in FIG. 2a.
FIGS. 3a-3c present three typical profiles of isolated or widely spaced metal lines formed over a substrate. FIG. 3a shows an isolated line 22a with a substantially vertical sidewall, FIG. 3b shows an isolated line 22b with a sloped sidewall, and FIG. 3c shows an isolated line 22c with a reentrant sidewall. The lines 22a-22c, respectively, are formed over substrates 24a-24c, respectively. Oxide or other layers 26a-26c are formed over the respective lines 22a-22c, respectively.
The metal line 22a with vertical sidewalls is not the optimal configuration for isolated lines. This is because the oxide layer 26a conforms to the shape of the line 22a and thus does not present a planar surface for subsequent layers. The metal line 22c suffers from the same problem as the metal line 17c, namely the formation of voids in layer 26c, such as voids 27. The optimal shape for isolated lines is therefore the sloped sidewall of line 22b as shown in FIG. 3b. As illustrated, layer 26b overlying the sloped metal line 22b provides a more planar surface for any subsequent layering, and thus, the sloped sidewall of line 22b is preferred for isolated metal lines.
With the optimal sidewall shape differing for densely packed and isolated lines in the metal layer, the problem exists in consistently producing the preferred line shape on the same die. What is needed is a process that can produce both the vertical sidewalls optimal in dense metal lines and the sloped sidewalls optimal in isolated metal lines on the same die.